This invention relates to integrated semiconductor devices and methods of making the same. In particular, the invention relates to devices incorporating optical waveguide structures as well as methods of manufacturing the same.
Description of the Related Art
Integrated optic devices are well suited to applications in such technologies as telecommunications, instrumentation, signal processing and sensors. An integrated optical circuit employs optical waveguides to implement devices, such as optical transmitters and receivers, switches and couplers. Waveguides also efficiently transmit light through the optical circuit and connect to external optical waveguides such as optical fibers, generally butt-coupled to the device. However, a mode mismatch exists between the semiconductor waveguide and the optical fiber. The former has typically a 1-2 xcexcm elliptical modal spot, which is neither well-sized nor shaped to match the standard 8-9 xcexcm circular modal spot of conventional single-mode optical fibers. Specifically, because the difference of the refractive index between the core and cladding of a typical waveguide is higher than that of a typical fiber, the optical field is more confined in the waveguide than in the fiber. In addition, waveguide core dimension is smaller than the fiber core dimension. Therefore, directly butt-coupled devices present 7-10 dB insertion loss. Nonintegrated solutions improve this coupling but present submicron alignment tolerances. To achieve both low coupling loss and large alignment tolerances, it is necessary to transform the mode on-chip to better match the fiber.
One first known solution is proposed in xe2x80x9cTapered waveguide InGaAs/InGaAsP multiple quantum well lasersxe2x80x9d by T. L. Koch, U. Koren, G. Eisenstein, M. G. Young, M. Oron, C. R. Giles and B. I. Miller, IEEE Photon. Technol. Lett., Vol. 2, No. 2, February 1990. This document describes a semiconductor laser emitting a wide guided mode. A transition using a stepped change in vertical thickness of waveguide is shown. This laser is however complex to implement and fabricate.
A second known solution is proposed in xe2x80x9cEfficient coupling of a semiconductor laser to an optical fiber by means of a tapered waveguide on siliconxe2x80x9d by Y. Shani, C. H. Henry, R. C. Kistler, K. J. Orlowsky and D. A. Ackerman, Appl. Phys. Lett., Vol. 55, December 1989. The structure described in this document avoids any complex vertical tapering, however, it does require a regrowth to define a large rib over the substrate from which a first grown small rib has been removed. It further requires separate lateral definition for the large and small ribs.
A further solution is proposed in EP 0545820 A1 by P. Doussixc3xa8re, and in xe2x80x9cTwo-dimensional control of mode size in optical channel waveguides by lateral channel taperingxe2x80x9d by R. N. Thurston, E. Kapon and A. Shahar, Opt. Lett., Vol. 16, no. 5, March 1991. These documents describe a set of two optical rib waveguides superposed over at least part of their lengths, the transverse cross-section area of the upper waveguide decreasing in a mode transition section to couple its narrow optical mode to a broad mode which is guided by the underlying rib waveguide. This device requires one growth step but two separate lateral definition steps for the two rib waveguides, the last one involving critical alignments.
This invention aims to provide a widened output mode semiconductor optical device, which is simple to implement, is easy to fabricate and which results in only low losses of light.
This invention also aims to provide a method of manufacture of a widened output mode semiconductor optical device, which is simple to implement, is easy to fabricate and which results in only low losses of light.
According to a first aspect, the invention provides a waveguide component having a slab waveguide, a rib waveguide, and a mode transition section wherein the rib waveguide and the slab waveguide are adjacent, and wherein the rib waveguide is tapered, to provide optical coupling, and lateral confinement waveguides are provided along the slab waveguide in the mode transition section to confine light from spreading laterally. The lateral waveguides may also be rib waveguides. A rib waveguide preferably comprises a lower confinement layer having a first refractive index, a core layer having a second refractive index which is higher than the surrounding materials, an upper cladding layer having a refractive index lower than the index of the core, and a material that generally flanks the core and the cladding layers. Generally, the material of the upper cladding layer is the same as the lower confinement layer. The material flanking the cladding and the core may be the same as the cladding. In this case, the rib core is buried in the material that forms the cladding and the surrounding material.
The lateral confinement rib waveguides are significant for enabling a mode transition section which couples with low loss, which reduces the fabrication difficulties of prior art arrangements, and which is easy to implement. In particular, it reduces the need for the taper to be very fine, or the taper to be made in both vertical and horizontal axes, both of which cause fabrication difficulties.
As preferred additional features, the lateral confinement ribs can be formed in the same plane as the tapered rib, they can be formed coaxially and along both sides of the tapered rib, they can be formed on the slab, and they can be formed in the same manufacturing step as the tapered rib. If the same manufacturing step is used, there is no need for difficult precise alignment of separate steps, if a single mask is used for example. The tapered rib can be shaped so that the tapering becomes finer as the rib gets narrower. The slab waveguide can have vertical confinement by an antiresonant reflection layer having a refractive index higher than a core of the slab waveguide.
Another aspect of the invention can be summarized as a rib waveguide grown on a slab waveguide, the width of the rib waveguide decreasing in a mode transition section in which said rib waveguide is sided by two sets of confinement waveguides, grown in the same fabrication step as the central rib waveguide. The decrease in the central rib waveguide results in the coupling of its optical mode to a wider mode guided by the slab waveguide and laterally confined by the influence of the lateral waveguides.
Other aspects of the invention include methods of manufacturing the waveguide component, and applications to optical components such as semiconductor lasers, waveguide arrays for filtering, for dispersion compensation, for wavelength division multiplexing/demultiplexing, and switching, and arrays of receivers for example. This recognizes the great value that the waveguide component can bring to such applications. In particular, in wavelength division multiplexed systems, there may be tens or hundreds or more of optical paths, and so providing compact and less expensive interfaces to integrated waveguide components can be commercially valuable. Other suitable applications are semiconductor optical amplifiers, optical regenerators, optical filters, optical splitters.
How the present invention may be put into effect will now be described with reference to the appended schematic drawings. Obviously, numerous variations and modifications can be made without departing from the spirit of the present invention. Therefore, it should be clearly understood that the form of the present invention is illustrative only and is not intended to limit the scope of the present invention.